The present invention relates to a hydrodynamic test apparatus.
Just as there are wind tunnels for the aerodynamic testing of ground or air equipment, there are hydrodynamic test apparatus for research on the behavior of partially or totally submerged mobile equipment.
Certain of these apparatus include a tunnel delimiting a liquid vein set in motion by appropriate means. There must be a considerable difference, however, between the diameter of the liquid vein and the maximum diameter of the model to be tested. Otherwise, measurements on the model will be distorted by the tunnel walls those presence modifies the flow of liquid in the vein. To obtain this considerable difference, one can use a vein of large dimensions, but it is very difficult to ensure a steady flow of water over the entire cross section of such a vein. Moreover, such facilities are predicated on moving a very large quantity of liquid and are, therefore, very expensive. It is also possible to use very small-scale models, but then the correspondence of the model to the original is rather low and the tests therefore are not useful for practical operations.
Therefore, test facilities have been contemplated, in particular for nautical keels, in which a test model is mounted on a support. The support is generally an overhead crane which moves over a tank of standing water. Equipment of this type is very satisfactory for conducting model tests of mobile equipment which is partly submerged and partly not submerged. In this case, it is possible to attach a model to a support at the non-submerged part of the model and the movement of the support does not cause any disturbance around the submerged part of the model, which is the only part being tested.
Such types of apparatus do, however, have drawbacks for research on totally submerged models such as models of submarines or models of towed equipment such as a buoy. The support must be sufficiently rigid to avoid oscillating or vibrating under the effect of the flow of water. Such a high degree of rigidity assumes that the support will be rather large. The support will thus distort the range of speeds around the model, first, because of the volume of water displaced, and secondly, because of the disturbances created at the level where the support traverses the surface of the fluid.
Given these difficulties and considering that research models very often have places of symmetry not intersected by any velocity vector components, it was thought that satisfactory modelling would be obtained by moving a symmetrical plane half-scale model mounted on a plate which is essentially parallel to the bottom of a tank. It is turned out to be very difficult, however, to achieve steady movement of a submerged platform whose resistance to forward movement is very high and is modulated by the influence of the sea state simulated in the tank.
In particular, towing a test stand by means of a cable proved ineffective because of the elasticity of small cables and the excessive disturbances caused by thick cables. It also proved impossible to move a platform holder by means of a motor-driven rollers because the rollers would slip on the bottom of the tank or along their support rails. Finally, the towing of a submerged platform was contemplated by means of battery-driven electric motors in the manner of torpedoes. Structures of this type, however, have not proved satisfactory due to the short life and bulkiness of the batteries, which disturbs the flow of water around the model.